A planned and started transfer to the decarbonized power grids is based first of all on a significant increase in a share of non-fossil and renewable (mainly wind and solar) energy sources in global electricity generation. According to the Blue Map scenario of “Prospects for Large-Scale Energy Storage in Decarbonized Power Grids”, Working Paper, IEA 2009, contribution of ‘green” power in total energy balance should grow up to 11-12% by 2050. However with the large shares of renewable energy, it may be desirable to take steps to ensure the on-demand and reliable supply of electricity, taking into account a variable output of the renewable energy sources and a frequent both positive and negative unbalance between this output and a current demand for power. One of the possible ways for solving this problem is the use of large-scale energy storages in the decarbonized power grids. According to the mentioned IEA estimates, an installed capacity of such energy storages should be increased from 100 GW in 2009 up to 189-305 GW by 2050. The large-scale energy storages could also solve a problem of operating the base-load (mainly coal and nuclear) power plants without significant reduction in the output of their steam generators during off-peak (low demand for power) hours in electrical grids.
Amongst the known methods for energy storage able to accumulate a lot of energy and store it over a long time-period, the methods for Liquid Air Energy Storage (LAES) (see e.c. Patent FR 2,489,411) are distinguished by a much simpler permitting process and the freedom from any geographical, land and environmental constraints, inherent in such other methods for large-scale energy storage technologies as Pumped Hydro Electrical Storage (PHES) and Compressed Air Energy Storage (CAES). In such LAES systems liquid air is produced using excessive power from the grid, stored in the small volume tanks between the off-peak and on-peak hours and used as effective working medium in the periods of high power demand. One of the most important properties of any electric energy storage is its grid Round Trip Efficiency (RTEGRID). When durations of energy storage charge and discharge are equal, the RTEGRID is defined as a simple relationship between a power produced and delivered to the grid during storage discharge (WDCH, kW) and power consumed from the grid during storage charge (WCH, kW), that is to say as RTEGRID=WDCH/WCH. A power produced may be determined as WDCH=ωDCH×GLA, where ωDCH—specific power produced by one ton of liquid air per hour (kW/(ton/h)) and GLA—hourly flow-rate of liquid air (ton/h) during LAES charge and discharge. In its turn, a GLA value may be determined as GLA=WCH/ωCH, where ωCH—specific power consumed for production of 1 ton of liquid air per hour (kW/(ton/h)). It is evident that the values GLA, WDCH and eventually RTE may be primarily increased through reduction in a ωCH value. Below is analyzed this way for of the LAES RTE improvement.
The most simple one turbo expander-compressor based air liquefaction process which may be used in the LAES system compresses a mixture of feed and recirculating air to one pressure level, cools the compressed mixed air, and work expands a recirculating portion of it in one turbo expander to provide the refrigeration for another portion of air, which is liquefied. The cooling effect produced by this work expansion step is defined as auto-refrigeration. The remaining portion of the compressed gas is cooled in the heat exchanger against the recirculating air stream, reduced in pressure, and recovered as a liquid. A more detailed description of the one turbo expander-compressor based air liquefier is presented in the Patent Application No. DE 10,2012,104,416 and U.S. Pat. Nos. 5,836,173 and 6,230,518. From the latter description it is clear that air liquefaction ratio (ALR) in such liquefier does not exceed 12%, resulting in ωCH value well above 500 kWh/ton and unacceptably low LAES RTE value.
The use of multiple turbo expander-compressors which operate over different temperature levels and different or the same pressure levels provides air auto-refrigeration at the most appropriate locations of the heat exchanger, reduces a power consumed by the compressor(s) and improves the efficiency of the air liquefaction process and RTE of the LAES as a whole. Two turbo expander-compressors based air liquefiers may be exemplified by the U.S. Pat. No. 4,778,497, DE 10,147,047 and US Patent Application Pub. No. 2015/0192065. As evident from the mentioned patent documents, two turbo expander-compressors based process provides an increase in ALR value up to 13.5-17.5% and specific power consumption in bulky process in the region of ωCH=400-450 kWh/ton. However, under these conditions a LAES RTE value remains significantly below those values inherent in the competitive energy storage technologies (PHES and CAES).
A further considerable decrease in power consumed in air liquefaction process simultaneously with simplification of this process may be achieved through co-location of the LAES and Liquefied Natural Gas (LNG) import (regasification) terminal. In this case a vast cold thermal energy potential of the regasified LNG could be profitable used for an effective cooling of compressed air, as it is proposed in the UK Patent Application No. GB 2,512,360 and U.S. Provisional Application Nos. 62/105,411, 62/267,433, 62/548,982 and 62/550,704. Our estimates show that replacement of two turbo expander-compressors scheme by LNG cold recovery may increase the ALR value up to 70-88%, whereas a specific power consumption may be reduced down to ωCH=110-190 kWh/ton. However, a possibility for co-location and integration of the LNG import terminal and LAES is of infrequent occurrence, resulting in some restrictions of this method applicability.
The other possible ways for improvement in LAES performance are based on extracting a cold thermal energy from the liquid air being re-gasified during LAES discharge, storing this energy in the cold storage and its recovery in the process of air liquefaction during LAES charge. Cold storage may provide all the LAES demands for refrigeration in the process of air liquefaction as described in the Patent Applications No. US 2012/0,216,520, US 2013/0,240,171, US 2015/0,192,330, US 2015/0,218,968, EP 2,930,318, as well as WO 2015/154,894 and U.S. provisional patent application No. 61/955,156. In these cases the ALR value at the stand-alone LAES facility with adiabatic compression of charging air may be increased up to 95%, whereas an energy consumption in the air liquefaction process may be reduced down to ωCH=160-170 kWh/ton. However, a great deal needs to be done before a workable and reliable cold storages with direct contact between a bed of solid thermal energy storing material and highly-pressurized charging and discharged air undergoing cyclic liquefaction and regasification in the wide range of the temperatures from cryogenic to atmospheric values will be designed. It is expected that this design will be distinguished by the enhanced mass and dimensions and restrictions on relationship between the pressures of charging and discharged air streams, while construction and operation of such storages should be conducted in accordance with the strong regulations. The use of liquid cold storing material with an indirect contact between it and treated air eliminates certain of the mentioned problems, but results in origination of the new problems, like a high cost of the used liquid material, its explosion and fire hazard and a need for an additional equipment (heat exchangers, pumps, tanks, piping, etc.) of such cold storage.
The attempts at improvement this situation led to development of the hybrid LAES schemes, wherein a part of cold capacity required for air liquefaction is provided through use of one or two turbo expander-compressor based air auto-refrigeration, whereas a rest of required cold is extracted from the cold storage (see UK Patent Application No. GB 2,494,400 and US Patent Applications No. 2011/0,132,032 and 2015/0,192,358, EU Patent Applications EP 2,880,268, EP 2,976,511 and EP 2,835,506). As compared to the two turbo expander-compressor based liquefiers such hybrid LAES provide an increase in LAES ALR value by 20-30 points up to 37-47% and make possible to reduce specific power consumption down to ωCH=200-240 kWh/ton. However the improvements in the hybrid LAES performance are leveled down by its enhanced complexity and first cost, which are particularly high when employing the integration of two turbo expander-compressor based liquefier and cold storage with indirect contact between the solid or liquid cold storing material and treated air.
By this means simultaneously with the ongoing attempts at further decrease in the ωCH level, new approaches are called for increase in amount of liquefied air GLA and correspondingly in WDCH value without any increase in power WCH consumed from the grid or co-located power plant during LAES charge which may be limited for one reason or other. This is possible through co-location of the LAES facility with any source of thermal energy at an enhanced temperature and conversion of this energy QTH into an additional compression power, used for increase in a GLA value at the LAES facility. The harnessing a local thermal energy in lieu of external electrical or mechanical energy for operation of the cryogenic refrigeration apparatus has been suggested in the U.S. Pat. No. 3,214,938. However the technical solution described in this patent is not applicable to design of the LAES facility for a number of reasons and above all owing to: a) in the analyzed solution production of cold and conversion of thermal energy into power are performed in two different closed loops, whereas in the LAES facility the deep air cooling and its liquefaction with possible harnessing a thermal energy should be performed in one open loop; b) there is a drastic difference in temperature drop of the refrigerant in the heat absorber of closed loop cryogenic refrigeration apparatus and of the process air in the open loop liquefier (44K and 204K); c) there is a crucial distinction in phase state of single phase gaseous refrigerant and liquefied/re-gasified air; and d) there is a significant difference in the technologies of one and two turbo expander-compressor based gas auto-refrigeration.
A method for two turbo expander-compressors based air auto-refrigeration and liquefaction, including the operation of both expanders at the same pressure drop and different temperature levels and exemplified by the U.S. Pat. No. 4,778,497, may be best adapted for recovery of thermal energy from any co-located source. However, such integration calls for introducing a number of the significant changes into such method aiming to simplify it and increase a round-trip efficiency.
In addition, a thermal energy of co-located external source may be profitable used also for increase in specific power (ωDCH) produced by the re-gasified air during the LAES discharge. Together with the mentioned increase in GLA value this makes possible to significantly increase an output power WDCH of the LAES and correspondingly enhance a RTE value of the latter. However this calls for development of the special means for more effective harnessing an available assistant thermal energy during LAES discharge.
Finally, there is a need for a further increase in the WDCH and RTE values of the LAES facility at the sacrifice of a more rational use of cold thermal energy released by the re-gasified process air stream. For these purposes the known principle of so-called “cold” and low energy-intensive compression of an additional air, as a working medium for the closed bottoming or main open power cycles is proposed to use in the Patent Applications No. US 2012/0151961 and US 2015/0192065. In both cycles the energy consumed in the process of “cold” compression of an additional air is reduced significantly, resulting in a greater amount of an additional power produced. However, both the mentioned technical proposals are distinguished by an enhanced complexity and the high first costs of an additional air production and usage. It is expedient to simplify a harnessing of cold thermal energy released by the re-gasified air stream in the LAES facility.
As a whole, the method for liquid air energy storage including one or two turbo expander-compressor based air auto-refrigeration and liquefaction with operation of both expanders at the same pressure drop and different temperature levels is selected as a subject for an innovative improvement in the present invention. Thereby, a harnessing of thermal energy from any external, but co-located source in the processes of the LAES charge and discharge is found to be an effective means for achievement of the invention's goals. In addition, a more rational method of harnessing a cold thermal energy released by the re-gasified air stream for a further increase in the WDCH and RTE values of the LAES facility should be developed.